In the design of most devices consideration is given to the final size and weight of a device. For many devices, there is a desire to minimize the size and/or weight of the device, particularly for portable electronic devices such as mobile phones and unmanned vehicles such as drones. Often the energy source for a device, e.g. a battery, a capacitor, an electrochemical capacitor, a fuel cell or a bank of such energy sources, contributes a substantial portion of the size and weight of the portable device. Thus, there it would be desirable to reduce the final size and/or weight of the energy source or bank of energy sources that is included in devices, particularly portable devices.
It has been proposed to reduce the size and weight of portable devices by making various components of such devices multifunctional. The energy source, typically a battery, is often one of the largest or heaviest single components in an electrical device and, thus, is one which would benefit most from being multifunctional. One example of multifunctionality is to add mechanical stiffness or load-carrying capacity to an energy storage device. Attempts have been made to produce multifunctional batteries which contribute to the mechanical strength of a device in which the battery is included in U.S. patent application Ser. No. 11/594,863 filed on Nov. 9, 2006 titled “Structural Batteries and Components Thereof”; in U.S. Provisional Patent Application Ser. No. 60/735,999 filed on Nov. 9, 2005 titled “Structural Composite Battery” and in “Design and Processing of Structural Composite Batteries” Proceedings of SAMPE 2007 Baltimore, Md. 3-7 Jun. 2007 by James F. Snyder et al. which is hereby incorporated by reference herein. Attempts have also been made to produce multifunctional fuel cells which contribute to the mechanical strength of the device in which the fuel cells are included in U.S. patent application Ser. No. 11/594,861 filed on Nov. 9, 2006 titled “Structural Fuel Cells and Components Thereof” and in U.S. Provisional Patent Application Ser. No. 60/735,991 filed on Nov. 9, 2005 titled “Structural Composite Fuel Cell”. And, attempts have also been made to produce multifunctional capacitors which contribute to the mechanical strength of the device they supply in U.S. patent application Ser. No. 11/594,862 filed on Nov. 9, 2006 titled “Structural Capacitors and Components Thereof” and U.S. Provisional Patent Application Ser. No. 60/735,998 filed on Nov. 9, 2005 titled “Structural Capacitors”. It would also be desirable to provide multifuctional electrochemical capacitors that could contribute to the mechanical strength of the device to which they supply energy to reduce the size and/or weight of the device.
Structural electrochemical capacitors would be more (or in addition to) advantageous than structural batteries, structural capacitors and structural fuel cells because electrochemical capacitors can provide higher specific power than batteries or fuel cells and higher specific energy than capacitors (c.f. Ragone plot in FIG. 3 in “What Are Batteries, Fuel Cells and Supercapacitors?” Chem Rev 2004, 104, 4245-4269 by Martin Winter et al.). Electrochemical capacitors are particularly useful to provide rapid charge or discharge capabilities in combination with high specific energy devices such as batteries or fuel cells or energy harvesting devices such as photovoltaics. One useful application is for charge conditioning in which a structural electrochemical capacitor stores energy from other sources for load balancing purposes and provides an appropriate charge routine to a high specific energy device such as a battery. Structural electrochemical capacitors also have potential for long life, theoretically up to millions of charge-discharge cycles, due to the relative absence of electrode swelling and interfacial degradation in comparison to devices such as batteries that may only last for hundreds or perhaps thousands of charge-discharge cycles. Advantageously, structural electrochemical capacitors also have little danger of overcharging, a common problem with batteries.
The structural energy storage devices described herein, structural electrochemical capacitors, contribute both energy storage and mechanical strength to a system or device in which the electrochemical capacitors are incorporated. The mechanical strength is owing to two factors: the design of matted or woven-laminate electrodes based for example on activated carbon fibers, and a resin or polymer electrolyte which impregnates the electrodes. The available energy density may be increased by using carbon fibers with a high surface area achieved through surface treatment or activation. Power densities greater than that obtained from batteries are realized by more rapid capacitive charge storage mechanisms such as arise from the electrical double-layer at the electrode surfaces.
Batteries, capacitors and electrochemical capacitors of the type used for delivery of relatively high levels of power or energy are generally fairly large bulky items, and their size and weight can impose a significant design constraint on systems, particularly portable systems, in which they are included. Heretofore, these size and weight constraints have limited the utility and practicality of particular systems. As will be described in detail herein below, the present invention recognizes that by the appropriate selection of materials, electrochemical capacitors may be manufactured which combine good structural properties such as strength and stiffness together with good electrical properties. Such electrochemical capacitors can be used to form structural elements of systems in which they are incorporated, and hence these electrochemical capacitors are referred to herein as “structural electrochemical capacitors.” Since structural electrochemical capacitors function both as capacitive storage devices as well as structural components of systems, the weight and/or size burden imposed upon such systems by the need for capacitive storage is greatly minimized. For example, portions of the air frame of an unmanned aerial vehicle or other vehicle may be fabricated from one or more structural electrochemical capacitors. Hence, the overall weight of the vehicle will be reduced and/or the amount of electrical power carried by the vehicle can be significantly increased. It will be appreciated that similar power/weight/size benefits will be achieved with regard to other devices such as cellular telephones, other communications equipment, computers, microsystems and specialized electronic devices and systems. In such devices, the structural electrochemical capacitors of the present invention can function as circuit boards, housings, casings, protective members and so forth. Further details and advantages of the present invention will be apparent from the drawings, discussion and description which follow.